The lin-12 gene of C. elegans is the archetype of the “lin-12/Notch” gene family found throughout the animal kingdom (reviewed in Greenwald and Rubin, 1992). Members of this family appear to function as receptors for intercellular signals that specify cell fates during development. Essentially, lin-12 activity controls binary decisions: if a cell has a choice between two fates, A and B, activation of lin-12 above a threshold value causes the cell to adopt fate A, whereas the failure to activate lin-12 above the threshold causes the cell to adopt fate B (Greenwald et al. 1983). Furthermore, inappropriate activation of mammalian lin-12/Notch genes have been implicated in oncogenesis (Ellisen et al., 1991; Robbins et al., 1993) and in normal development (e.g. Swiatek et al., 1993). Much of the work in applicants' laboratory is focused on understanding how lin-12 specifies cell fates. An important component of this endeavor is the identification of genes that influence lin-12 activity and the identification of potential “downstream” genes.
Applicants identified the sel-12 gene by screening for suppressors of the “Multivulva” phenotype caused by an allele of lin-12 that causes constitutive LIN-12 activation. Applicants performed a genetic and molecular characterization of sel-12, which established: (1) Reducing or eliminating sel-12 activity reduces the activity of lin-12 and of glp-1, another member of the lin-12/Notch family. In addition, reducing or eliminating sel-12 activity causes and egg-laying defective (Egl) phenotype. Applicants do not know if the Egl phenotype is a direct consequence of reducing lin-12 activity or an independent effect of reducing sel-12 activity. (2) sel-12 and lin-12 can functionally interact within the same cell. (3) sel-12 is predicted to encode a protein with multiple transmembrane domains that is highly similar to S182, which has been implicated in early-onset familial Alzheimer's disease (Sherrington et al., 1995). These findings have been described in a paper that has been accepted by Nature (Levitan and Greenwald, 1995). In addition, applicants have data indicating that sel-12 is more broadly expressed than lin-12, including a lot of expression in neurons.
The remarkable conservation of the SEL-12 and S182 predicted protein structure suggests that their functions are likely to be conserved as well. Recently, a second gene known as E5-1 or STM2 has been implicated in early-onset familial Alzheimer's disease (Levy-Lahad et al, 1995; Rogaev et al, 1995) E5-1/STM2 encodes a protein that is highly similar to S182 (Levy-Lahad et al, 1995b; Rogaev et al, 1995) and SEL-12. Furthermore, it is striking that four of the five changes in S182 or E5-1/STM2 associated with early-onset familial Alzheimer's disease alter amino acids that are absolutely conserved in the worm and the human proteins, and that the tenth alters an amino acid that has been changed very conservatively during evolution. Applicants hope to bring the powerful tools of classical and molecular genetic studies in C. elegans to bear on fundamental issues of SEL-12/S182/E5-1 structure and function. Thus, far, proteins similar to LIN-12 and SEL-12 have not been described in single-celled organisms, so C. elegans may be the simplest practical system for studying these issues in vivo.